1. Field of the Invention
This invention relates to a method and apparatus for the measurement of attenuation and delay time distortion (group transit time distortion) in a test object to which a test pulse is supplied.
2. The Prior Art
It is known to test objects such as transmission lines or the like by applying a test pulse to one end of the object under test, and analysing the pulse when it is received at the output of the test object. At the output, the deformation of the test pulse is analysed by Fourier analysis, the output signal being split into harmonics to determine the individual frequency components in terms of their amplitudes and their phase relationships to each other. The test pulse consists of a series of components having different frequencies, each component having a predetermined amplitude and starting phase. The test pulse is formed in accordance with the following: ##EQU1## where A.sub.n is the amplitude and .theta..sub.n is the starting phase of each individual component. When the individual components are superimposed to form the test pulse, the crest factor is ##EQU2## the crest factor is reduced, so that the object under test is not overloaded, by the selection of different starting phase values .theta..sub.n, in accordance with the method described in my application Ser. No. 088,091 filed Oct. 24, 1979, now U.S. Pat. No. 4,275,446, which is hereby incorporated by reference into this specification. That application deals with the problem of how the crest factor of the test pulse can be reduced by the selection of suitable starting phase values for the components of the test pulse. This is necessary in order to reduce the loading of the object under test as far as possible, to prevent overmodulation or other nonlinearities which could occur during the measuring process. When a test pulse with a relative high crest factor is used, a fault can be produced in the object under test which does occur in practice during the transmission of useful signals.
The method described in application Ser. No. 088,091 is carried out under the principle that the test pulse is optimized at the transmission end of the object under test, by selecting initial phase values for the components of the test pulse to yield the minimum crest factor. In practice, however this method does not lead to optimum results, because the object under test (such as a communications transmission line) alters the phases of the components to a greater or lesser degree, such that at the receiving end, the test pulse no longer contains the same relationships of the starting phase values, but instead other phase values which can lead to an undesired increase in the crest factor.